Variable frequency oscillator



Sept. 25, 1951 DE WlTT R. GODDARD VARIABLE FREQUENCY OSCILLATOR Filed June 9, 1948 INVENTOR DE WITT R. GODDARD ATTORNEY Patented Sept. 25, 1951 VARIABLE FREQUENCY OSCILLATOR De Witt Rugg Goddard, River-head, N. Y., assignor to Radio Corporation of America, a corporation of Delaware Application June 9, 1948, Serial No. 31,941

Claims.

The present invention comprises an improved oscillator, and particularly a variable frequency oscillator having circuits for controlling the frequency thereof by means of piezo-electric crystals.

Heretofore, it has been customary. to use either a variable oscillator or a crystal controlled oscillator. Attempts to combine the variable oscillator with crystal controlled features have not been successful in the past because of the 001m plicated switching circuits employed which tended to detract from the stable and reliable operation of the oscillator as an adjustable unit.

The foregoing difliculties are overcome by the oscillator of the present invention which employs only a simple switch having a single arm and a multiplicity of contact positions. One of these contact positions of the switch connects directly to ground, while the other contact positions of the switch enable the selection of any one of two or more crystals to be used.

A detailed description of the oscillator of the invention follows, in conjunction with a drawing whose single figure illustrates two triode electron discharge devices II and I2. Tube H has an anode A, a grid G and a cathode K. Tube H! has an anode A, a control grid G and a cathode K. Anodes A and A are connected to opposite sides of a two-terminal parallel tuned circuit composed of an inductance 2 and a variable tun- The anode of tube I2 is coning condenser 3. nected to the control grid of tube H by coupling condenser i. This control grid of tube H is maintained at substantially ground potential by resistor 5. connected together by lead I5 and are connected to ground through common cathode resistor 6, in turn, bypassed for radio frequency energy by compensating condenser E. The positive terminal of a source of unidirectional anode voltage +B is connected to the anode of tube H by lead l6 and bypassed to ground by condenser I. This same positive terminal supplies anode voltage to tube l2 through inductance 2. The control grid of tube i2 connects to the movable arm of a single pole multi-throw (multi-contact position) switch 9 and to resistor 8. Two of the contacts of switch 9 have associated therewith separate piezo-electric crystals IE] and [3 having different frequencies of operation. One terminal of resistor 8 is connected to ground. Resistors 5 and 8 are grid leaks and have a value of the order of 1 megohm. Although separate vacuum tubes have been shown, it should be understood that both triode electrode structures can be enclosed The two cathodes of these tubes are ma single evacuated envelope, and that, if desired, multi-grid tube structures can be employed instead of trio des.

When the arm of switch 9 is in the left hand or ground position, the oscillator becomes a variable frequency oscillator. The voltage which is applied to grid G of tube H is fed to the cathode. circuit of tube [2. Utilizing the cathode circuit of tube ll as an output circuit results in inverse feedback and the voltage applied to the grid G of tube H appears unamplified on itscathode. Due to this high degree of inverse feedback, the grid of tube H may be swung over a wide range of voltages without drawing grid current so that the impedance presented to the tuned circuit 2, 3' by the grid is that of a negative biased or class A'grid which is very high. In order to produce sustained oscillations, the feedback voltage fed from the tuned circuit 2, 3 is fed through the grid- G of tube I! through blocking condenser 4. This feedback voltage must be fed back tothe grid G of tube l2 with phase reversal in order to produce oscillations. This is readily accomplished due to the fact the common oath-- ode resistor 6 effects such reversal. A positive voltage on the grid G of tube I l causes more current to flow through the cathode resistor 6 and. thereby makes both cathodes K and K more positive. The cathode K of tube I2 becoming more: positive is equivalent to the grid G becoming more negative. Thus, a positive change on the grid G of tube I l is equivalent to a negative voltage change on grid G of tube i2 and the required. 180 phase reversal is obtained. Tuned circuit 2,. 3 is tuned to the required frequency of oscillation.

In one embodiment of the variable oscillator which was capable of being continuously tuned in a frequency range between 5 and 10 megacycles, it was found that condenser greatly improved the constancy of the radio frequency output volt-- age.

Now if, by means of condenser 3, this oscillator is tuned to a frequency somewhat higher than thenominal frequency of a piezo-electric crystal l3 and switch 9 turned so as to connect piezo-- electric crystal l3 to the control grid of tube l2, the oscillator will be controlled by this crystal. Ina similar manner, another piezo-electric crystal It may be utilized or in fact, any number of piezo-electric crystals of different frequencies may be selectively inserted into the circuit, one at a time, merely by connecting the arm of switch 9 to the particularcontact position in circuit with the crystal to be selected for controlling v the frequency of the oscillator. When a crystal,

3 is selected by the movement of the arm of switch 9, the precaution should be observed that the tuned circuit consisting of inductor 2 and condenser 3 be adjusted to a frequency somewhat higher than the nominal frequency of the crystal selected and to be used.

In the oscillator of the invention actually tried out in practice and which was tunable over a frequency range between 5 and megacycles, it was found that this frequency adjustment higher than the nominal frequency of the crystal is not critical. Any setting of this frequency between a few hundred cycles above and a couple of megacycles above the crystals nominal frequency will provide crystal control of the oscillator.

The term ground employed herein is deemed to be a point of reference potential and is not limited to an actual earthed connection.

What is claimed is:

1. An oscillator comprising first and second electron discharge electrode structures, each having an anode, a cathode and a grid, a variable frequency determining circuit having one terimal connected to the anode of said first structure and to the grid of the second structure, through a capacitor, a common resistor connecting said cathodes to a point of reference potential, means connecting the anode of the second structure to another terminal of said frequency determining circuit and to said point of reference potential for radio frequencies, a switch having a movable arm and a plurality of contacts, a direct current connection from said arm to the grid of said first structure, a direct connection from one of said contacts to saidpoint of reference potential, whereby said oscillator operates as a variable frequency oscillator when said arm is on said one contact, and a piezo-electric crystal connected between another of said contacts and said point of reference potential, whereby the frequency of said oscillator is controlled by said crystal when said arm is on said last contact.

2. An oscillator comprising first and second electron discharge electrode structures each having an anode, a cathode and a grid, a two-ter-' minal parallel turned circuit having a tuning element, connections from one terminal of said tuned circuit to the anode of said first structure and to the grid of said second structure through a capacitor, a resistor connecting the grid of said second structure to ground, a connection from the other terminal of said tuned circuit to the anode of said second structure, a connection of low impedance to radio frequency energy from the anode of said second structure to ground, a direct connection between said cathodes, a common direct current passing impedance between said last connection and ground, a bypass condenser in shunt to said common direct current passing impedance, a resistor connecting the grid of said first device to ground, a switch having a movable arm and a plurality of associated contacts adapted to be selectively engaged by said arm, a direct connection from said arm to the grid of said first structure, a direct current connection from ground to one of said contacts, whereby said oscillator operates as a variable frequency oscillator when said arm engages said one contact, and a crystal connected between another of said contacts and ground, said tuned circuit being tunable by said tuning-element to a frequency above the nominal frequency of said crystal, whereby said oscillator is controlled by said crystal when said arm engages said last contact.

3. An oscillator comprising first and second thermionic discharge device electrode structures each having an anode, a cathode and a grid, a tunable resonant circuit coupled to the anode of said first structure and to the grid of said second structure through a capacitor, separate means for effectively maintaining the anode of said second structure and the grid of said first structure at ground potential for radio frequency energy, one of said means including a switch having a plurality of individually selectable contacts, and a connection from one of said contacts to ground, a direct connection between said cathodes, a common direct current passing impedance between said direct connection and ground, and a. plurality of piezo-electric crystals having different frequencies of operation individually connected to different contacts on said switch other than said one contact, said tunable resonant circuit being tunable to frequencies higher than the nominal frequencies of said crystals.

4. An oscillator comprising first and second discharge device electrode structure each having an anode, a cathode and a grid, a parallel tuned circuit having a variable tuning element coupled between the anodes of said electrode structures, a capacitor coupling the anode of said first structure to the grid of said second structure, individual grid leak resistors between the grids of said electrode structures and ground, a bypass condenser coupling the anode of said second structure to ground, a common cathode resistor bypassed by a capacitor connecting said cathodes to ground, a source of anode polarizing potential coupled to the anode of said second structure, whereby theanode of said first structure is supplied with said polarizing potential through said tuned circuit, a switch having an arm directly connected to the grid of said first structure, a plurality of selectable contacts for association with said arm, a direct connection to ground from one of said selectable contacts, and a piezo-electric crystal connected between another of said selectable contacts and ground.

5. An oscillator comprising a pair of vacuum tube electrode sections each having an anode, a cathode and a grid, a resonant circuit coupled between said anodes, means for varying the effective frequency of said resonant circuit, means for capacitively coupling the grid of one of said sections to said resonant circuit, means coupling said cathodes together, an impedance connected from said cathodes to ground, a plurality of piezoelectric crystals of different frequencies, and switching means connected to the grid of said other section for selectively coupling any desired one of said crystals thereto.

DE WITT RUGG GODDARD.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS OTHER REFERENCES Wireless Engineer, November 1944, pages 521-26. 

